Conventionally, electrodeposited copper foil with carrier foil has been used as basic material for manufacturing printed wiring boards which are widely used in the fields of electric and electronic industries. In general, electrodeposited copper foils are bonded to glass-epoxy substrates, phenolic substrates, and insulating substrates made of polymers including polyimide, by means of hot pressing, to be utilized for manufacture of printed wiring boards as copper clad laminates.
In the hot pressing, copper clad laminates are obtained in such a way that copper foils, prepregs (substrates) cured to B-stage, and additional end plates to serve as spacers are laminated in multiple layers, and thermal compression is applied to bond the multiple layers of copper foils and prepregs under the high temperature atmosphere (hereafter, this process will sometimes be referred to as “pressing”). In this connection, if wrinkles are present on the copper foils, cracks are generated in the wrinkled portions in the copper foils, which sometimes causes such inconveniences that resin exudes from the prepregs, and wiring breaking in formation circuits occurs in the printed wiring board manufacturing process that is an etching process to be conducted subsequently.
Wrinkle generation in copper foils becomes a serious problem as the copper foils become thinner. Electrodeposited copper foils with carrier foil have won attention as such epoch-making copper foils that overcome the above described problems and can prevent the foreign matter contamination of glossy surfaces of copper foils in the time of hot pressing. In other words, electrodeposited copper foils with carrier foil have a configuration in which carrier foils and electrodeposited copper foils are bonded together in a planar shape, and are subjected to pressing with the carrier foils remaining attached, and the carrier foils can be removed immediately before the formation of copper circuits by etching. Thus, the prevention of wrinkle generation both at the time of handling and at the time of pressing of electrodeposited copper foils and the prevention of surface contamination of copper clad laminates become possible.
In the specification concerned, the designation of “carrier foil” is employed; the carrier foil is utilized in the form of being bonded to electrodeposited copper foils in a shape as close as possible to a planar shape. The “carrier foil” in the specification concerned has the following characteristics. A consideration of the manufacture method of the electrodeposited copper foil with carrier foil according to the present invention reveals that the carrier foil is required to have at least electric conductivity because copper component is electrodeposited on the carrier foil surface to form the electrodeposited copper foil. Besides, the electrodeposited copper foil with carrier foil is subjected to a sequential line of manufacturing processes, and has a role that meanwhile it maintains the state of being bonded to the electrodeposited copper layer at least until the end of the manufacture of copper clad laminates, facilitates the handling, and reinforces the electrodeposited copper foil in all senses; accordingly, the carrier foil is required to have a prescribed strength. Any object that satisfies the above described requirements can be used as “carrier foil,” and accordingly, the designation of “carrier foil” is used as a concept widely encompassing conductive films and the like, although metal foils generally tend to be brought to mind as such objects.
The electrodeposited copper foil with carrier foil can be classified broadly into the peelable foil and etchable foil in general. To sum up the difference therebetween, the peelable foil is the one in which the carrier foil is removed by peeling after pressing, while the etchable foil is the one in which the carrier foil is removed by etching after pressing.
The conventional peelable foil, however, is extremely unstable, after pressing, in the values of the peel strength of the carrier foil, and the satisfactory range of peel strength is generally assumed to fall in the range from 50 to 300 gf/cm. On one hand, in extreme cases, it transpires that the carrier foil cannot be peeled and there has been a drawback that it is difficult to obtain targeted peel strengths. The drawback has been the most serious obstruction when the electrodeposited copper foil with carrier foil widely prevails to be used in general purposes.
The cause for the fact that the peel strength of the carrier foil becomes unstable can be interpreted as described below. Conventional electrodeposited copper foils with carrier foil have adopted the structure in which an adhesive interface layer, based on such a metal as represented by zinc, is formed between a carrier foil and an electrodeposited copper foil, irrespective as to whether the peelable foil is concerned or the etchable foil is concerned. Additionally, the choice between the manufacturing of the peelable foil and the manufacturing of the etchable foil has been conducted practically by controlling the amount of metal that is made present in the adhesive interface layer, although there are slight differences depending on the carrier foil types.
The formation of a metal based adhesive interface layer is mainly conducted by the electrolytic deposition based on the electrolysis of a solution containing a prescribed metal element, and thus the electrochemical methods have been adopted. The electrochemical methods, however, find difficulty in extremely small quantity control of electrolytic deposition amount, and accordingly inferior to other technical methods in reproducibility. Furthermore, because the necessary electrolytic deposition amount boundary for the bifurcation between the peelable foil and the etchable foil is, in other words, the small difference in the metal amount present in the adhesive interface layer, it is conceivable that it is difficult to bring forth the stable performance. Furthermore, in general, peeling of the carrier foil is conducted, on completion of the press operation carried out at a temperature of 180° C. or higher, under a high pressure exerted, and for 1 to 3 hours, and accordingly it is possible that the adhesive interface layer gives rise to the mutual diffusion with the carrier foil and electrodeposited copper foil. This rather operates toward enhancing the adhesive strength, and is possibly part of the reason that the peel strength becomes unstable.
In these years, demand for downsizing electric and electronic appliances steadily continues to grow, and accordingly it has become strongly demanded that printed wiring boards as fundamental parts for these appliances be made in multilayer structure, the copper foil circuits thereof be made in high density, and packaging parts be mounted in high packaging density. To meet these demands, heat release from the packaging parts is increased, and hence printed wiring boards as built-in parts are required to have high heat resistance so that high heat resistant substrate including substrates utilizing BT resin, Teflon substrates, polyimide substrates have come to be used. In this connection, generally speaking, the press temperature for copper clad laminates as material for printed wiring boards also tends to become higher. Consideration of the above described matters leads to an anticipation that the use of conventional peelable electrodeposited copper foils with carrier foil will become more and more difficult.
On one hand, in connection with the etchable foil, dedicated etching equipment is needed, and the temporal cost required for etching tends to be high. Consequently, the use of conventional etchable electrodeposited copper foils with carrier foil results in boosting up the overall manufacture cost, and accordingly there has been a drawback that the conventional etchable foils cannot easily be used for an extremely wide range of applications.
The inventors involved in the application concerned, for the purpose of overcoming the above described problems, have proposed and supplied to the market an electrodeposited copper foil with carrier foil which eliminates the drawbacks of conventional peelable electrodeposited copper foils with carrier foil, makes the use thereof possible in a comparable manner with that in the case of the so-called usual use of electrodeposited copper foils, also forms an adhesive interface layer on the carrier foil surface so that the peel strength associated with the interface between the carrier foil and the electrodeposited copper foil can be stabilized to a low level, makes copper deposit electrolytically on the adhesive interface layer, utilizes the deposited copper layer as electrodeposited copper foil, and makes the above described adhesive interface layer by use of organic material (hereinafter, referred to as “electrodeposited copper foil with carrier foil provided with organic adhesive interface layer”).
The electrodeposited copper foil with carrier foil, provided with an organic adhesive interface layer, however, is given a relatively low heat resistance temperature for the organic material constituting the organic adhesive interface layer, and the heat resistance temperature of the organic material generally usable for forming the organic adhesive interface is generally limited to the order of 200° C. Therefore, under the existing circumstances, it has been difficult to use organic material for manufacturing Teflon substrates, polyimide substrates and the like, all requiring the press processing at the temperatures higher than the above mentioned limit.
Decomposition of the organic material constituting the organic adhesive interface layer by heating causes the mutual diffusion between the carrier foil and the electrodeposited copper foil, preventing smooth peeling, which has made impossible the use of the electrodeposited copper foil with carrier foil for the purpose of applications expected by nature thereto.
Additionally, as a technique involving a copper foil with carrier foil that allows easy peeling of the carrier foil even after such high temperature pressing, there is proposed independent use of a metal oxide layer for the adhesive interface layer between the carrier foil layer and copper foil layer, as disclosed in EP-1133220. This is a positive application of the characteristics borne by metal oxide that metal oxide is a compound stable at high temperatures and generally hard but brittle to the adhesive interface.
Incidentally, independent use of a metal oxide layer causes the following drawback constituting an obstruction that prevents pursuing the mass productivity involving finished products and improving the production yield. That is, when the thickness of the metal oxide layer exceeds a certain thickness, the peel strength between the carrier foil layer and the copper foil layer is increased, and additionally the peel strength comes to lack stability, and accordingly it becomes difficult to guarantee the quality of peel strength between the carrier foil layer and the copper foil layer.